Abstract

The lower toggle of an main landing gear had failed prematurely during the qualification test. Examination revealed that there was multiple fatigue crack initiation at the wide end of the toggle which subsequently gave rise to two major fatigue crack fronts around the bore. These cracks have propagated by fatigue to about 10-20 mm depth before giving way to static overload fracture. Fatigue crack initiation was found to be associated with shrinkage cavities/porosity in the toggle material. These defects acted as stress concentrators for multiple fatigue crack initiation in the component. The material was found to meet the specification 3.4394 T7451 with regard to chemical composition and heat treatment. A detailed analysis of the failure is presented in this report.